The documentation suggests using setContentScaleFactor() if you want to have different sets of assets for different resolutions. The problem is that this applies to the whole game so you have to include HD and SD versions of every sprite you want to use.

I want to have HD versions of some specific, important graphics and use the SD version for the rest (which is more than enough). Is there some method/technique to achieve this? Something like, for instance, assigning a scale factor to each different search path?

I am sure someone has this figured out since I it seems like a very bad idea to be forced to have HD and SD versions of every sprite in a large project